For example, US 2002/0011240 A1 (JP-A-11-107820)), discloses control of a combustion state in a cylinder of a compression ignition engine such as a diesel engine by manipulating an amount of exhaust gas recirculation (EGR), which returns into the cylinder. Further, a combustion control, in which EGR is increased to reduce emission of NOx, is developed so as to comply with a tightened emission regulation. However, combustion in a cylinder may become unstable when EGR is increased, and consequently a combustion time point may vary relative to a target value. For example, when the combustion time point, which indicates a combustion state, varies, emission cannot be controlled at a desirable exhaust state. Further, an application of an engine of a low compression ratio, which is capable of reducing emission of NOx, is also researched. However, compared with an engine of a normal compression ratio, fuel burns under a condition of low cylinder temperature in an engine of a low compression ratio. Therefore, combustion in the cylinder becomes unstable, and a combustion time point varies relative to a target value. In such an engine, the combustion time point may further vary due to change in environmental conditions such as fuel quality, intake-air temperature, or atmospheric pressure.
For example, JP-A-11-125141 discloses detection of cylinder pressure using a pressure sensor, detection of the ignition time point as an index, which indicates a combustion state, based on the cylinder pressure, and control of an ignition time point at a target value by manipulating the fuel injection time point based on the detected ignition time point. However, in a specific engine operation state or an injection pattern, the ignition time point cannot be detected based on the cylinder pressure. More specifically, when fuel injection quantity is small or pilot injection is close to main injection in a multi-stage injection, the ignition time point is not detectable based on the cylinder pressure.
It is conceived to detect a mass fraction burned 50 time point (MFB50 time point), at which a combustion mass fraction in a cylinder becomes 50% of total in one burning cycle, as an index, which indicates a combustion state of the ignition time point, based on the cylinder pressure. In this case, the MFB50 is controlled at a target value by manipulating the fuel injection timing. The MFB50 is detected by calculating a summation of a combustion mass in one burning cycle based on cylinder pressure. The MFB50 is a combustion time point at which the summation becomes 50% of total of the combustion mass and detectable based on the cylinder pressure, regardless of a specific engine operation state or an injection pattern. The MFB50 is detected from the summation of the combustion mass. Therefore, the MFB50 is apt to be influenced by detection error of the pressure sensor, and hence lower in detection accuracy compared with the ignition time point. Thus, the combustion state is hardly controlled based on one index such as the ignition time point or the MFB50, which has a detection characteristic with merits and demerits.